Human activity has likely been the most dominant cause of global warming since the mid-20th century, says an international report issued in Sweden Friday.

Warming in the climate system is undeniable, the U.N. Intergovernmental Panel on Climate Change report said.

Since 1950, changes have been observed throughout the climate system that are unprecedented, it said.

Each of the last three decades has been successively warmer at Earth's surface than any preceding decade since 1850, said the report, "Summary for Policymakers of the IPCC Working Group I assessment report, Climate Change 2013: the Physical Science Basis."

The Nobel Prize-winning panel's report was approved Friday by member governments of the IPCC in Stockholm.

"Observations of changes in the climate system are based on multiple lines of independent evidence. Our assessment of the science finds that the atmosphere and ocean have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased," said Qin Dahe, the working group's co-chairman.

Thomas Stocker, the other co-chairman, said sustained emissions of greenhouse gases would cause more warming and changes in all aspects of the climate system. Limiting climate change would require "substantial and sustained" reduction of greenhouse gas emission, he said.

"Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5 degrees [Celsius] relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2 degrees [Celsius] for the two high scenarios," Stocker said. "Heat waves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions."

The Working Group I report assessed global and regional-scale climate change for 21st century.

"As the ocean warms, and glaciers and ice sheets reduce, global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years," Dahe said.

The report found that ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90 percent of the energy accumulated between 1971 and 2010.

"As a result of our past, present and expected future emissions of CO2, we are committed to climate change, and effects will persist for many centuries even if emissions of CO2 stop," Stocker said.

"This is yet another wake-up call: Those who deny the science or choose excuses over action are playing with fire," U.S. Secretary of State John Kerry said. "Once again, the science grows clearer, the case grows more compelling, and the costs of inaction grow beyond anything that anyone with conscience or common sense should be willing to even contemplate."

Kerry said the report's essence was: "Climate change is real; it's happening now; human beings are the cause of this transformation; and only action by human beings can save the world from its worst impacts."

He said the report wasn't meant to be filed away or produced by politicians.

"It's science," Kerry said. "It builds on the most authoritative assessments of knowledge on climate change produced by scientists, who by profession are conservative because they must deal in what is observable, provable and reviewable by their peers."

"If this isn't an alarm bell, then I don't know what one is."

The United States is "deeply committed" to taking the lead on climate change, working with its partners to reduce emissions, transform the energy economy and help the most vulnerable regions cope with the effects of climate change, Kerry said.

The group's report will be part of assessments by two more working groups that will be released next March and April. The IPCC Fifth Assessment Report cycle concludes with the publication of its Synthesis Report in October 2014.

Friday's report is the culmination of studies by more than 850 scientists from 85 countries whose work was compiled over three years by about 250 authors from 39 countries. That work was reviewed by more than 1,000 experts.

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